X-ray computerized tomograph including collimator that restricts irradiation range of X-ray fan beam

ABSTRACT

An X-ray CT apparatus including a collimator that restricts an irradiation range of an X-ray fan beam, in which first projection data is obtained with no restriction placed onto the X-ray irradiation range through the collimator, then, second projection data is obtained with the X-ray irradiation range restricted in accordance with a concerned region set on a tomographic image, and when reconstructing an image with the use of the second projection data, the image is reconstructed using, as data outside of the concerned region, data of a corresponding portion of the first projection data.

TECHNICAL FIELD

The present invention relates to an X-ray CT (computerized tomograph)that obtains an in-body tomographic image of a patient with the use ofan X-ray. More particularly, it relates to an X-ray CT that, whencontinuously measuring almost the same cross section, suppresses theX-ray irradiation toward a region outside a set concerned region up tothe smallest possible degree, thereby making it possible to reduce anX-ray exposure dose toward the patient, an operator or a specifictissue.

BACKGROUND ART

The X-ray CT has already been used widely in fields such as medicalcare. For example, in recent years, when endermically executing biopsyof a nidus or the treatment thereof, the X-ray CT is used as the guideto a puncture. Executing, in this way, the biopsy of a nidus or thetreatment thereof under the guide by the X-ray CT is now considered tobe an effective and helpful method, since this method can be expectednot only to shorten an operation time but also to enhance accuracy ofthe operation.

By the way, for guidance by the X-ray CT, there exist two methods. Onemethod is a method in which the puncture and the CT scanning arerepeated alternately and intermittently, while confirming data such as aposition of tip of a puncture needle. The other method is a method inwhich the CT scanning is performed continuously and the image issequentially displayed so that the position of tip of the punctureneedle can be confirmed immediately (CT fluoroscopy). In particular, thelatter method permits the tomographic image to be obtained on a realtime basis, thus bringing about an advantage of shortening the operationtime even further.

However, in the methods where the CT scanning is executed intermittentlyor continuously as are described above, an increase in the X-rayexposure dose, which is caused by the intermittent or the continuous CTscanning, has become a problem to the patient or the operator. The X-rayexposure dose is determined by the X-ray irradiation dose and slicewidth set by a slice collimator. In order to reduce the exposure dose,it is sufficient to decrease the irradiation dose just by decreasing atube electric current passing through an X-ray tube. The decrease in theirradiation dose (mAs=mA×sec), however, means an increase in the noiseby X-ray fluctuation. This, accordingly, has resulted in a problem thatpicture quality of the tomographic image has been deterioratedexceedingly.

DISCLOSURE OF THE INVENTION

Then, in view of the above-mentioned problems in the conventionaltechniques, it is an object of the present invention to provide an X-rayCT that is capable of reducing the X-ray exposure dose toward thepatient or the operator without causing the deterioration of picturequality of the obtained tomographic image even if the X-ray photographyis executed continuously or intermittently at the time of operationssuch as the above-described puncture based on guidance by the X-ray CT.

According to the present invention, in order to accomplish theabove-mentioned object, an X-ray CT is provided that rotates an X-raysource continuously to measure projection data of a subject continuouslyover a plurality of times and, based on the projection data,reconstructs a tomographic image of the subject to sequentially displayit on a display, including a concerned region setting unit that sets,within the subject, an irradiation range of an X-ray emitted from theX-ray source, and an X-ray shielding apparatus, i.e. a channelcollimator, that, when rotating the X-ray source to measure theprojection data, restricts an irradiation range of a fan beam so as tosuppress X-ray irradiation outside of the concerned region set by theconcerned region setting unit.

Also, in the present invention, there is provided an image processorthat, when a measurement is performed with the X-ray fan beam convergedon the concerned region, makes it possible to reconstruct a tomographicimage in a region with the use of measurement data obtained previously.The region exists outside of the set concerned region, and the channelcollimator suppresses the X-ray irradiation toward the region. The imageprocessor also displays the tomographic image in the region.

Further, in the present invention, in the course of said plural times ofcontinuous measurements, the channel collimator is so controlled as toobtain projection data acquired by normal photography (globalmeasurement scanning) or projection data in a region that is wider ascompared with the concerned region, the normal photography being notrestricted to the X-ray irradiation region set by the concerned regionsetting unit.

Moreover, in the present invention, the projection data are measured byirradiating the subject with X-rays while gradually reducing orexpanding the X-ray irradiation range between the concerned region setby the concerned region setting unit and the global measurement scanningrange.

Furthermore, in the present invention, there is provided an imageprocessor that is capable of reconstructing projection data in a regionoutside of the concerned region through extrapolation from scanned dataobtained by irradiating only the concerned region with X-rays.

In addition, in the present invention, the concerned region setting unitis configured to display on the display a boundary between the insideand the outside of the set concerned region.

Namely, in the present invention, the concerned region setting unit andthe channel collimator make it possible to execute the photography insuch a manner that the X-ray irradiation toward a region other than theconcerned region is suppressed up to the smallest possible degree.Moreover, an image in the region outside of the concerned region isreconstructed by fitting and embedding previously measured data and soon.

As is obvious from the above-described detailed explanation, accordingto the X-ray CT in the present invention, in operations such asre-inspection and a CT fluoroscopic photography, it is possible toreduce ineffective and needless X-ray exposure by employing localmeasurement scanning. In the local measurement scanning, only a rangeobtained by restricting in advance a portion to be photographed (aconcerned region) is irradiated with X-rays. Also, as to data in aregion other than the concerned region, embedding of data that areprevious in time makes it possible to reconstruct an image with fewerartifacts. Namely, these characteristics make it possible to obtain,with a low X-ray exposure dose and concerning the image outside of theconcerned region as well, a precise and high picture quality image thathas a relatively small difference in time.

In addition, the present invention makes it possible to reduce the X-rayexposure dose toward a subject or the operator without decreasing thetube electric current. This characteristic permits a high accuracydiagnosis or operation to be performed without deteriorating picturequality of the image. Also, regarding the boundary between the insideand the outside of the concerned region, the boundary has been indicatedclearly on the image. This is a measure to be taken to prevent amisdiagnosis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an entire configuration diagram of an X-ray CT in the presentinvention;

FIG. 2 is a detailed illustrative diagram of a scanner system in theX-ray CT in the present invention;

FIGS. 3A, 3B are detailed illustrative diagrams of a channel collimatorin the present invention;

FIGS. 4A, 4B are diagrams illustrating operation of a shielding leadboard in the channel collimator;

FIGS. 5A to 5C are diagrams showing a relation between a convergenceposition of the channel collimator and X-ray irradiation;

FIG. 6 is a geometrically illustrative diagram of the channelcollimator;

FIG. 7 is a diagram illustrating scanning of a concerned region by theX-ray CT in the present invention;

FIG. 8 is a diagram showing an embodiment of flow of the photography inthe X-ray CT in the present invention;

FIG. 9 is a diagram showing an embodiment clearly indicating a boundaryof the concerned region;

FIG. 10 is a diagram explaining flow of image reconstruction accordingto the present invention;

FIGS. 11A to 11C are diagrams explaining data embedding in the presentinvention;

FIG. 12 is a block diagram illustrating an example of an image processorin the X-ray CT in the present invention;

FIGS. 13A to 13C are diagrams explaining an example of a correctingmethod for data outside the concerned region;

FIG. 14 is a diagram explaining another example of the correcting methodfor the data outside the concerned region;

FIG. 15 is a diagram showing another embodiment of the flow of thephotography in the X-ray CT in the present invention;

FIG. 16 is a diagram showing another embodiment of the flow of thephotography in the X-ray CT in the present invention;

FIG. 17 is an illustrative diagram of reduction/expansion photography ofthe concerned region in the embodiment in FIG. 16;

FIG. 18 is a diagram explaining the image reconstruction at the time ofthe reduction photography;

FIG. 19 is a diagram explaining the image reconstruction at the time. ofthe reduction/expansion photography in the embodiment in FIG. 16; and

FIG. 20 is a diagram showing another embodiment of the flow of thephotography in the X-ray CT in the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Using the accompanying drawings, the detailed explanation will be givenbelow concerning embodiments in the present invention.

First, FIG. 1 shows an entire configuration of an X-ray CT according toan embodiment in the present invention. As is apparent from the drawing,the X-ray CT includes a display 100, a host computer 101 for controllingthe entire apparatus, a scanner system 102 mounting thereon systems suchas an X-ray generating system and an X-ray detecting system and allowinga continuous scanning with the use of a slip ring, an image processor103 being in charge of a pre-processing of an image, an imagereconstructing processing or various kinds of analyzing processings andincluding a preamplifier (indicated by reference numeral 106 in FIG. 2),a high voltage generator 104 for feeding a high voltage to the X-raygenerating system, a patient table 105 for mounting a subject thereon, atable controller 107 for controlling a position in x, y, z directions ofthe patient table 105, a slice collimator (108 in FIG. 2) forrestricting X-ray irradiation range to a slice width, a channelcollimator (210 in FIG. 2) for restricting X-ray irradiation range in afan beam direction, a collimator controller 211 for controlling aconvergence position of the channel collimator, and so on. Incidentally,although not illustrated, the host computer 101 includes as its inputapparatuses a keyboard, a mouse, a tracking ball, and so on.

FIG. 2 is a detailed illustrative diagram of the above-mentioned scannersystem 102. According to the configuration of the X-ray CT in thepresent invention, in proximity to an X-ray tube (an X-ray source) 200and at the same time between the X-ray tube and a subject B to beinspected, the channel collimator 210 is installed. The X-ray tubegenerates an X-ray with the use of the high voltage fed from the highvoltage generator 104. Moreover, the channel collimator 210 iscontrolled by the collimator controller 211 in such a manner as to movein an x direction as shown in the drawing. Namely, the channelcollimator 210 is configured so that it can restrict X-ray irradiationrange in a channel direction to a concerned region A that is settable inadvance. Also, reference numeral 250 in the drawing denotes the X-raydetecting system in the scanner system 102.

In addition to the channel collimator 211, there is provided the slicecollimator 108. The slice collimator is a collimator for determining aslice width and thus it differs from the channel collimator. In general,both of the collimators are used in combination.

Next, FIGS. 3A, 3B and FIGS. 4A, 4B illustrate a schematic structure ofthe channel collimator 210 that characterizes the present invention. Asis illustrated in FIG. 3A, the channel collimator 210 includes acompensation filter 213 inside a collimator case 212 that istrapezoid-shaped in the outside appearance, and also includes ashielding lead board 214 at the base portion thereof. Also, FIG. 3Bshows the structure of a lower surface portion of the channel collimator210. Pair of guides 215 are provided on the both sides of theabove-described shielding lead board 214. Extension or contraction ofthe shielding lead board 214 is controlled by working operations of amotor 216 equipped with devices such as an encoder and a timing belt217. Incidentally, FIG. 4A and FIG. 4B show a state where the shieldinglead board 214 is contracted and a state where it is extended,respectively.

Also, as is illustrated in the above-mentioned drawing, the collimatorcase 212 is further provided with the collimator controller 211directing and controlling the whole channel collimator 210 and a motordriver 218 driving the above-mentioned motor 216. The collimatorcontroller 211 and the motor driver 218 may be installed on a memberother than the collimator case 212.

Speaking of the above-described collimator controller 211, at the timeof, for example, setting the concerned region A in the presentinvention, as its first step, several types of tables are stored in thecollimator controller. Also, the above-described motor driver 218 drivesand rotates the motor 216. Then, the rotation of the motor istransmitted to the timing belt 217. This allows the timing belt 217 tocause the curtain-shaped shielding lead board 214 to be extended orcontracted. This extension or contraction makes it possible to carry outthe scanning of the concerned region during the scanning as a whole.

Additionally, FIGS. 3A, 3B, 4A, and 4B illustrate just one example, and,naturally, there can be the other structure of the channel collimatorand the other controlling mechanism therefor.

FIGS. 5A to 5C show a relation among collimators 401, 402, X-ray fanbeams, and the detector 250.

FIG. 5A shows a position relationship among the collimators 401, 402(which correspond to the shielding lead board 214 in FIG. 3A) for theoriginal and normal CT measurement, i.e. the global measurementscanning, an X-ray fan beam A, and the detector 250. Meanwhile, FIG. 5Bshows a position relationship among the collimators 401, 402 for a CTmeasurement at a certain view angle (a projection angle), an X-ray fanbeam B, and the detector 250. Moreover, FIG. 5C shows a state at anotherview angle. In FIG. 5A, the collimators 401, 402 are in their referencepositions. Meanwhile, FIG. 5B shows an example where the collimator 401is displaced to the right by ΔXL₁, and the collimator 402 is displacedto the left by ΔXR₁. Moreover, FIG. 5C shows an example where thecollimator 401 is displaced to the right by ΔXL₂ and the collimator 402is displaced to the left by ΔXR₂. In the present invention, displacementdata ΔXL_(j), ΔXR_(j) for each view j are stored as a table in thememory, and then the displacement data are read out for each view angleso as to achieve the position control.

Here, using FIG. 6, the explanation will be given below concerning thecontrol of the channel collimator 210.

First, as is illustrated in FIG. 6, a straight line, which is drawn froma focal point position (Xs, Ys) of the X-ray tube, i.e. the generatingsource of an X-ray, to a center of rotation O of the X-ray tube, istaken as a y axis, and a straight line that intersects the y axis atright angles at the center of rotation O is taken as an x axis.

Here, the mechanism of the collimators 401, 402 is such that they aremovable in the x direction as described earlier, thereby, as is alsoillustrated in FIGS. 5A to 5C, making it possible to arbitrarilyrestrict the X-ray irradiation range for each view.

Now, in FIG. 6, assuming that the concerned region is defined by acircle with its center as a central coordinate (Xc, Yc) and having aradius r, it turns out that the X-ray irradiation range thus set rangesfrom θL to θR as an angle of viewing the focal point in perspective.Accordingly, displacement amounts of the collimators 401, 402 fromnormal positions on the x axis become ΔXL, ΔXR in the drawing. Here, thenormal position of the collimators 401, 402 are defined as XL₀, XR₀,while a coordinate of the focal point of the X-ray is defined as (Xs,Ys), considering the focal point to be a point focal point as describedabove. Then, it turns out that a position coordinate of the collimators401, 402 (XL₀−ΔXL, XR₀−ΔXR) varies, depending on each projection angle.

Incidentally, speaking of means of determining ΔXL, ΔXR, i.e. thedisplacement amounts of the collimators 401, 402 from the normalpositions, ΔXL, ΔXR can be obtained by the following calculatingformulae and thus let's determine them beforehand here. Also, instead ofusing the following calculating formulae, it is possible to make theapproximations using periodic functions such as trigonometric functions.

Namely, ΔXL, ΔXR are determined in the following way: $\begin{matrix}{d = \sqrt{( {{Xs} - {Xc}} )^{2} - ( {{Ys} - {Yc}} )^{2}}} & (1)\end{matrix}$

Here, d is an inter-two point distance between the focal point and thecentral coordinate of the concerned region. Representing θL and θR withthe use of d, the resultant formulae are as follows: $\begin{matrix}{{\theta \quad L} = {{\sin^{- 1}\frac{r}{d}} - {\tan^{- 1}\frac{Xc}{d_{0} - {Yc}}}}} & (2) \\{{\theta \quad R} = {{\sin^{- 1}\frac{r}{d}} + {\tan^{- 1}\frac{Xc}{d_{0} - {Yc}}}}} & (3)\end{matrix}$

Wherein d₀ is a distance between the focal point of the X-ray and acenter of rotation of the scanner. Representing ΔXL, ΔXR with the use ofθL and θR, the resultant formulae are as follows:

 ΔXL=XL0−y0 tanθL  (4)

ΔXR=XR0−y0 tanθR  (5)

In accordance with ΔXL, ΔXR determined above, positions of the channelcollimators 401, 402 are controlled for each projection angle. This, asis illustrated in FIG. 7, permits only the concerned region A of thesubject to be inspected to be irradiated with X-rays. Incidentally,although, in the above-mentioned example, the explanation has been givenassuming that the concerned region is the circle, the configuration ofthe concerned region is not limited to a circle. It may be the otherconfiguration such as, for example, an ellipse. In that case, parametersare added to the below-described procedures of determining ΔXL, ΔXR.

If the above-described movements of the collimators are carried outcontinuously, i.e. the collimators move during one view measurement aswell, there are actually some cases where edge of the X-ray irradiationrange does not completely coincide with boundary of a detecting elementin the detector 250 at the time of measuring each projection data.However, since a method described later makes it possible to decide aneffective channel, accuracy of the displacement amounts of thecollimators need not be so high. Depending on the cases, it is possibleto make approximations of the controlling parameters using periodicfunctions such as a sinusoidal wave.

Next, the explanation will be given below concerning flow of thephotography in the X-ray CT the configuration of which has beenexplained above. Incidentally, when a portion that the operator wishesto photograph is recognized in advance, the X-ray CT in the presentinvention is usable for, for example, reinspection after the operation,biopsy of a tumor through the CT fluoroscopy, and so on. The photographyprocedure will be explained below, using FIG. 8. Namely, in thephotography procedure, only a concerned region, i.e. the portion thatthe operator wishes to photograph, is irradiated with X-rays, therebyembodying lowering of the X-ray exposure dose in the X-ray CT.

Referring to the flow of the photography, when the photography isstarted, a scanogram (a fluoroscopic image that is photographed with theX-ray tube at rest and with the patient table 105 being moved) isdisplayed first (step 41), and next a decision on a precise photographyrange is made (step 42).

In the decision on a precise photography range, after setting thepatient on the table 105 is over, in order to decide a photographyposition of the tomographic image, the above-described scanogram isobtained first. Moreover, setting the number of the photographs isperformed on the displayed scanogram. The photography range is an itemrelated to items such as a photography starting, a photography intervaland the number of the photographs. For example, in the case of helicalscanning, items such as the photography starting position, atable-moving speed and the number of the scanning are also set.

After that, a precise photography is executed (step 43). In the precisephotography, in accordance with the conditions set above, the hostcomputer 101, for example, sets a tube electric voltage and a tubeelectric current to the high voltage generator 104, and also sets to thetable controller 107 a moving speed at the time of the helical scanningand so on. At the time of the precise photography, the above-describedchannel collimators 210, 401, 402 are in their normal positions, andthus the X-ray is let into all the channels. This permits a sufficientlydiagnostic image to be obtained in the subsequent reconstruction of aprecise image (step 44).

Subsequently, a decision on a photography slice (step 45) and a decisionon a concerned region (step 46) are executed. Namely, when theabove-described precise photography is finished, in the photographyslice, the operator observes the photographed image and, in the case of,for example, the reinspection and so on, selects a slice suitable for aportion that he or she wishes to observe most. Also, in the case of theCT fluoroscopy, the operator obtains information on the periphery of atarget tissue (a tumor), such as a position of the target tissue orwhether or not there exists an important tissue on a puncture route upto the target tissue, and decides a puncture slice at the time of the CTfluoroscopy. Furthermore, in the decision on a concerned region, theoperator sets a range to be irradiated with X-rays on the slice decidedabove, i.e. the concerned region A. Additionally, as is illustrated in,for example, FIG. 9, the setting of the concerned region is carried outby describing, for example, a circular region or an elliptic region on adisplay screen of the display 100, using pointing devices such as amouse and a tracking ball as input apparatuses. At the same time, X-rayirradiation toward outside the set concerned region is suppressed up tothe smallest possible degree. In addition, the table is displaced in theslice direction up to the set photography slice position (step 47).

Next, in a reference scanning (step 48), the data are measured one timeunder the same conditions as those in a continuous photography to beperformed thereafter. Namely, in the reference scanning, following aninstruction by the host computer 101, the table controller 107 displacesthe table up to the selected slice position. Then, with an X-ray dosethat is appropriate for a photography purpose of the CT fluoroscopy suchas the re-inspection or the puncture, the photography where the channelcollimator 210 is in its normal position (the global measurementscanning) is executed.

Next, in a puncture slice position-confirming step 49, it is confirmedwhether or not the target tissue is contained within the slice. In aconcerned region-confirming step 50, by actually displacing the channelcollimators, it is confirmed whether or not the image in the concernedregion has been displayed in a good condition.

If a desired image is obtained by the reference scanning, whilecontrolling positions of the channel collimators 401, 402 so that thepositions thereof become the irradiation field of view (range), go tothe continuous scanning of puncture photography (step 51). Then, thephotography data are reconstructed sequentially so as to display atomographic image that is continuous in time (step 52).

Subsequently, the explanation will be given below concerning an imagereconstruction based on projection data obtained by irradiating only theconcerned region with X-rays.

In an image reconstruction, it is often requested to reconstruct animage in a region outside the concerned region. The explanation will begiven below concerning an example of the processing content for thereconstruction.

FIG. 10 shows the flow chart thereof. In FIG. 10, an offset correctionby a preamplifier dark current (F1), an X-ray variation correction (F2),and a packing processing (F5) as preprocessing for a log conversion anda radiation quality correction (F3, E4) are added. After that, the imagereconstruction is executed (F6), and then the image thus reconstructedis displayed (F7). Here, the packing processing means a processing inwhich a data range that is ineffective for the reconstruction isrestricted is determined and the data therein are replaced by previouslymeasured data, since the irradiation range is restricted.

Then, using FIGS. 11A to 11C, the explanation will be given belowconcerning projection data that are measured when the photography isperformed with the irradiation range restricted. FIG. 11B shows anoutput I₀ at each position of the detector in a state where there is nosubject to be photographed, i.e. in the case of photographing air.Positions of the channel collimators correspond to ia, ib. FIG. 11Cshows a detector output I at the time of performing the photography witha subject to be photographed inserted. FIG. 11A shows an output afterthe log conversion. As is indicated by a bold solid line in FIG. 11C,projection data in a range shielded by the channel collimators 401, 402become equal to substantially zero after the offset correction.Consequently, at the time of data calculation after the log conversionillustrated in FIG. 11A, since I/I₀ is an extremely small value, anoverflow takes place. At this time, if data outside the effective datarange of the projection data after the log conversion are set to bezero, extremely high frequency components occur at the boundary channelsia, ib, and are emphasized by a reconstructing filter. This causesartifacts to appear on the image. The measure for solving these problemsis the packing processing.

The boundary channels ia, ib can be determined easily by θL, θR of theformulae (2), (3). However, a reliability of the data in proximity tothe boundary channels is rather low, because, as described earlier, theedge of the X-ray irradiation range does not completely coincide withthe boundary of the detecting element and the reliability depends on thedisplacement accuracy of the collimators. Accordingly, it may bepossible to employ, a method of regarding inside data by severalchannels as effective considering a little margin. Otherwise, it is alsoadvisable to define ia, ib by performing a threshold processing towardthe data after the offset correction in FIG. 11C. The use of thethreshold processing makes it possible to easily determine the effectivedata range even if the displacement accuracy of the collimators issomewhat low.

After the effective data range has been decided, a processing isexecuted that fits data existing in an ineffective data range of dataobtained by the global measurement scanning. Here, the data obtained bythe global measurement scanning mean data measured previously with nocollimate in the channel direction (data measured at the positions ofthe collimators 401, 402 on the regular CT measurement).

Assuming that, in FIG. 11A, projection data to be processed are R (i, j)and previously measured projection data are P (i, j) (i.e., for example,projection data measured with no restriction by the channel collimatorsat the time of the puncture slice position-confirming scanning),projection data after the packing processing R′ (i, j) are given by thefollowing formula. Reconstruction of R′ (i, j) allows an image of theconcerned region to be obtained. Here, reference note i denotes channelnumber and j denotes projection number. $\begin{matrix}{{R^{\prime}( {i,j} )} = \{ \begin{matrix}{{{{P( {i,j} )}\quad i} \leqq {ia}},{i \geqq {ib}}} \\{{{R( {i,j} )}\quad {ia}} < i < {ib}}\end{matrix} } & (6)\end{matrix}$

The packing processing presents no problem in the case of completelyidentical slices. If the slices are shifted, however, it is consideredthat a difference in level is formed at joints between the twoprojection data, i.e. P and R. In the case where there occur suchnon-negligible difference in level, it is necessary to performprocessings such as a moving average or a weighted average toward datain proximity to the joints so that the two projection data are connectedsmoothly to each other.

FIG. 12 shows an embodiment in which a processing is executed thatembeds the previously measured projection data toward the projectiondata in the range shielded by the channel collimators.

In FIG. 12, the image processor 103, which includes a reconstructingarithmetic unit 11 and a weight-imposition image adder 12, produces anoutput to the display 100. Moreover, the reconstructing arithmetic unit11 includes a projection data memory 20, a preprocessing arithmetic unit21, a fan beam-parallel beam converter 22, a filter correctionarithmetic unit 23, and a back projection arithmetic unit 24. Theweight-imposition image adder 12 includes seven units of image memories10 (#1 to #7), seven units of weight coefficient multipliers 13(reference notations W₁ to W₇ denote weight coefficients), and an adder25.

Incidentally, the image processor 103 is configured so that, when ascanning rotation is a 1-second-per-rotation, the image processor iscapable of reconstructing one image in, for example, less than onesecond. In this case, partially reconstructed images are obtained in awidth of, for example, 30° one after another sequentially, and onereconstructed image can be obtained by adding a plurality of latestpartially reconstructed images. Accordingly, in the 30° width of angleupdating, 12-images-per-scond of reconstructed images can be obtained.

Also, the reconstructing arithmetic unit 11 obtains the reconstructedimage by performing a preprocessing made by the arithmetic unit 21, aconversion to a parallel beam made by the fan beam-parallel beamconverter 22, a filter correction processing made by the arithmetic unit23, and in addition a back projection arithmetic operation made by theback projection arithmetic unit 24. This reconstructing arithmeticoperation is not a 360° amount of batch-type reconstructing arithmeticoperation but an adding arithmetic operation of the partiallyreconstructed images obtained from parallel beam data in a width of apartial angle (for example, in the width of 30°)

Furthermore, the weight-imposition image adder 12 allots one by one, toeach of the image memories 10, i.e. #1 to #7, the partiallyreconstructed images obtained one after another by the reconstructingarithmetic unit 11. For example, a partially reconstructed image g₁ isallotted to #1, a partially reconstructed image g₂ is allotted to #2, .. . , and a partially reconstructed image g₇ is allotted to #7, and thenthe partially reconstructed images are stored. Concerning the remainder,i.e. partially reconstructed images g₈, g₉ . . . , the allotment iscarried out in such a manner that g₈ is allotted to #1 instead of g₁, g₉is allotted to #2 instead of g₂, . . . , and then the storing isexecuted.

Making the weight coefficients, W₁ to W₇, correspond to the partiallyreconstructed images stored in the memories #1 to #7, respectively, theweight coefficient multipliers 13 multiply each of the images by each ofthe weight coefficients, thus performing a weight-imposition onto eachof the partially reconstructed images. The adder 25 executes a totaladdition thereof, thereby obtaining one reconstructed image.

FIGS. 13A to 13C show the concept of the above-described imagereconstruction processing performed by the image processor 103. Namely,the effective data range obtained by the global measurement scanning(GS) proves to be the range indicated by GS in a bold solid line in FIG.13A. In contrast to this, the projection data obtained by the localmeasurement scanning (LS) in the present invention, in which the X-rayirradiation is executed with the X-ray restricted only to the concernedregion, prove to be the data illustrated in FIG. 13B in the case of acontinues scanning, such as the helical scanning and a dynamic scanning,or in the case of repeating the CT fluoroscopy and so on. As is seenfrom LS in FIG. 13B, image data outside the concerned region A arediscontinuous in time or in space, thus scarcely allowing high picturequality to be accomplished.

Then, as illustrated in FIG. 13C, in the course of the above-describedcontinuous scanning or CT fluoroscopy, a scanning GS (global measurementscanning) where the channel collimator 210 is in its normal position isexecuted. Moreover, at the time of a continues local measurementscanning LS before or after this scanning, projection data obtained by aglobal measurement scanning that is the nearest thereto in time areembedded as correction data toward regions outside the concerned regionA. This transaction is capable of embodying an image reconstructionprocessing that makes possible lowering of the X-ray exposure dose andaccomplishing of the high picture quality toward the regions outside theconcerned region A as well. Namely, this makes it possible to obtain,with a low X-ray exposure dose, images that have a small difference intime between the regions inside and outside the concerned region A,thereby obtaining the image that is easier to see.

The above-stated explanation is about the case where the GS data canexist. In the case where there exist no Gs data, it is possible tocorrespond through extrapolation of the outside the concerned regionfrom the data inside the concerned region. FIG. 14 presents theexplanation thereof. As the extrapolation, simple least-squares methodmay be used, or a higher order interpolation such as Newtoninterpolation may be used.

In the case where the extrapolation is used, the discontinuous imagedata can also be avoided similarly in the case where the GS data isused.

When the subject extends off width of the detector, the extrapolationcan be used for reconstructing an image of the portion of the subjectthat extends off.

Also, as described earlier, the image outside the concerned region A isa high picture quality image (i.e. the projection data obtained by theglobal measurement scanning that is the nearest thereto in time) with adifference in time eliminated up to the smallest possible degree.Nevertheless, not a little difference in time exists between the imageoutside the concerned region A and the image inside the concerned regionA. On account of this, when applying the present invention to thecontinues scanning or the CT fluoroscopy, the operator is likely tomistake the image outside the concerned region A for the image insidethe concerned region A. It cannot be denied that this may result in hismisdiagnosis.

Then, in the present invention, as a measure of clearly indicating aboundary between the inside and the outside of the concerned region A,as illustrated in FIG. 9, the boundary line BL between the inside andthe outside of the concerned region A has been indicated clearly on thedisplayed image of the display 100, thereby preventing theabove-mentioned mistake and misdiagnosis. As the measure of clearlyindicating the boundary line BL, the following are appropriate: Theboundary line (a circle in FIG. 9) surrounding the concerned region A isdescribed with a line that, including a meaning of warning, is of aconspicuous color such as, for example, red. Otherwise, in the case of ablack-and-white monitor, a black or white line is used, or is blinkedthereby allowing the observer to recognize the boundary line easily.

FIG. 15 shows an embodiment of flow of the photography in which theglobal measurement scanning is executed in the course of the localmeasurement scanning. The steps other than a setting of a globalmeasurement-scanning interval (step 61) are the same as the steps in theflow illustrated in FIG. 8, and accordingly the explanation thereof isomitted.

In the setting of the global measurement-scanning interval at step 61,before proceeding to the continuous photography and so on, it is set howmany times the photography where the channel collimator 210 is in itsnormal position is executed in the course of the continuous photography.Incidentally, here, the more minutely the setting is made, the smallerthe shift in time of the image outside the concerned region becomes.Consequently, an image that becomes more precise by amount of thedecrease in the shift can be obtained, but the X-ray exposure dose isnot reduced very much. Also, the photography interval can also beautomatically set. In that case, a setting stored in advance in the hostcomputer 101, for example, a setting such as “One global measurementscanning is executed every 10 slices.” may be used without modification.Otherwise, instead of such automatic setting, a setting such as “Onenormal scanning is executed every 30 slices.” is also possible with theuse of a manual. A setting of “No global measurement scanning isexecuted in the course of the continuous scanning.” can also be made. Atthat time, the data obtained by the reference scanning executed at theabove-described step 48 are embedded toward the data outside theconcerned region, thereby executing the image reconstruction processing.

FIG. 16 shows an embodiment of flow of the photography in which thephotography is executed while moving the channel collimator. The samenumber is allotted to the same step as that in FIG. 8.

Namely, after the confirming of the concerned region (step 50), at astep 62, together with the setting of the global measurement-scanninginterval, it is further made possible to make a setting of expansionphotography of the concerned region. Namely, if a reduction/expansionsetting is selected here, between the concerned region photography andthe global measurement scanning, the concerned region A and the regionfor the global measurement scanning gradually come close to each other.In other words, it is possible to execute a scanning the measuring rangeof which is gradually reduced from the global measurement scanning rangeor a set range to the concerned region A.

Also, in the case of the expansion, the measuring range is graduallyexpanded from the concerned region A to the normal scanning region orthe set range.

Also, in the case of the reduction, assuming that the concerned region Ais of a circular shape, a circle of the global measurement scanningrange or the set range is gradually reduced and, finally, the measuringrange attains to the concerned region A (In the case of the expansion,the measuring range is altered in the reverse way.). Moreover, datafetched outside the concerned region A while reducing (or expanding) areused for the correction of the image reconstruction explained above,thereby making it possible to achieve the high picture quality of theimage in the region outside the concerned region A.

In the present embodiment, when selecting the reduction/expansionphotography, it turns out that a radius of the above-described circle ofthe concerned region A or a central coordinate of the concerned regionA, in some cases, is altered during the scanning. As is illustrated inFIG. 6, however, it is obvious that, under the control of the channelcollimator 210 according to the above-described embodiment, it isoperable when the central coordinate of the concerned region A isshifted from the center of rotation of the X-ray source.

However, making an assumption that, for simplicity, the centralcoordinate of the concerned region A coincides with the center ofrotation of the X-ray source, the explanation will be given belowconcerning the operation in the case where the reduction/expansionphotography is selected. Additionally, the assumption results in acondition that a value of the radius r of the concerned region A becomesan only parameter that is altered. FIG. 17 illustrates a photographysituation where, in this way, the photography range of the concernedregion A is gradually reduced (or expanded).

As illustrated in FIG. 17, in the course of a scanning such as thecontinues scanning or the fluoroscopy photography, a photography isexecuted with the channel collimator 210 gradually reduced from theglobal measurement scanning range to the concerned region A or graduallyexpanded from the concerned region A to the global measurement scanningrange. By this, a high picture quality can be obtained by embedding onlythe data in a region unobtainable by the local measurement scanning withthe projection data obtained by the global measurement scanning and asingular or a plurality of projection data obtained previously in time.

This processing will be explained below, using projection data at thetime when, for simplicity, the scanner 102 has been rotated one turn andthe X-ray source 200 is directly above the concerned region as isillustrated in FIG. 17. Namely, now, it is assumed that the X-rayirradiation range is gradually reduced from the normal scanning regionto the concerned region A for each of the turns of the scanner.According to this assumption, projection data obtained by the respectivescannings (i.e., the first scanning to the fourth scanning) are given asillustrated in FIG. 18.

Namely, in FIG. 18, the bold solid lines indicate the respectiveeffective data ranges. Concretely, first, the effective data range inthe first scanning is the widest, and, conversely, the effective datarange in the fourth scanning (a scanning for the concerned region A)becomes the narrowest. As to the data obtained in the second scanning,the projection data in regions shielded by the collimator arecompensated with projection data in the above-mentioned first scanning.Moreover, the projection data in the second scanning, including theabove-mentioned projection data embedded from the first scanning in theprojection data in the second scanning, are embedded in projection datain regions shielded by the collimator in data obtained by the thirdscanning. Furthermore, the projection data in the third scanning areembedded in data in the fourth scanning.

In this way, according to the present embodiment, the projection data inthe first and the second scannings are included in the projection datain the third scanning. Consequently, it turns out that the data inregions shielded by the collimator in the fourth scanning areconstructed by being compensated with the data in the first to thirdscannings. Namely, the above-mentioned image reconstructing proceduremeans that, using the data that are nearer in time, the data areembedded one after another. This allows the image outside the concernedregion A as well to be reconstructed as a high picture quality image.

Incidentally, the above-mentioned explanation in FIG. 18 has been givenconcerning only the case where the measuring region is reduced. Thepresent invention, however, is not limited thereto. As will be explainedbelow using FIG. 19, a method is also possible that executes the imagereconstruction based on both the reduction photography and the expansionphotography of the measuring region.

In FIG. 19, for explanation, straight lines represent projection dataobtained by the measurement. Namely, the bold solid lines in the drawingindicate the effective data ranges that are actually obtained (i.e., notshielded by the channel collimator), and dashed lines indicate the datain ranges that are not measured. In the explanation in FIG. 19, thefollowing assumption has been made: The first to the seventh scanningsare executed, and each of the scannings indicates projection dataobtained at the first projection angle in the first scanning, and themeasuring range thereof is altered for each of the scannings. Also, asis apparent from the drawing, the first and the seventh scannings areglobal measurement scannings, and the fourth scanning is a scanning foronly the concerned region A.

In these projection data, the procedure in image reconstructingprocessings based on the first to the fourth projection data is the sameas the procedure already explained, and accordingly the explanationthereof is omitted here. Moreover, in an image reconstructing processingin the fifth scanning in which an expansion scanning is executed, thedata in regions shielded by the collimator are compensated with the datain the second and the first scannings, and in an image reconstructingprocessing in the sixth scanning, it is compensated with the data in thefirst scanning. Also, although not illustrated, in the seventh scanningor thereafter, the image reconstructing processings are performed usingunits of the above-described combined data. By doing as above-mentioned,since the data that are nearer in time are employed and embedded oneafter another, a high picture quality image can be reconstructed for animage of regions other than the concerned region A.

FIG. 20 shows another embodiment of the flow of the photography in thepresent invention.

In the present embodiment, a displacement amount of the patient table isdetermined that allows the center of a set concerned region to coincidewith the center of rotation of the X-ray source, thereby making itpossible to easily set the center of the concerned region to be thecenter of rotation of the X-ray source. As illustrated in FIG. 20, inthe course of the flow of the photography in FIG. 8, there is provided afunction that the host computer (101, FIG. 1) and the table controller(107, FIG. 1) automatically displace the patient table in an x-ydirection (step 63). If the set field of irradiation is a circularregion the center of which coincides with the center of rotation of theX-ray source, even if a measurement is made at any angle, the viewingangles become equal to each other. Thus, it is enough to set thecollimator only once before the scanning. Consequently, if theirradiation range is determined, it is sufficient to execute positioncontrol of the table just one time. This makes the control easier andthe reliability higher. The packing processing also becomes easier sincethe effective data ranges become the same at the respective view angles.

The present invention is not limited to the embodiments disclosed abovebut includes a variety of modifications included in the claims.

INDUSTRIAL APPLICABILITY

The present invention is applicable to fields such as an x-ray-usedinspection or an x-ray photography in which it is desirable to lower thex-ray exposure dose.

What is claimed is:
 1. An X-ray CT apparatus, comprising: a displayapparatus displaying a tomographic image; means for setting a concernedregion on the tomographic image displayed by said display apparatus;X-ray shielding means for restricting an irradiation range of an X-rayfan beam in relation to said concerned region; means for controllingsaid X-ray shielding means so that said irradiation range of said X-rayfan beam through said X-ray shielding means and restriction positions ofsaid X-ray shielding means are controlled in response to a rotationangle of an X-ray irradiator; means for, through said X-ray shieldingmeans, irradiating with X-rays from said X-ray irradiator a rangeincluding said concerned region in a subject to be inspected andexecuting an X-ray CT measurement; and means for obtaining outside ofconcerned region data by means of calculation and reconstructing animage with the use of said outside of concerned region data and data ofthe irradiation range restricted by said X-ray shielding means, saidoutside of concerned region data being data of a range that is notrestricted by said X-ray shielding means.
 2. The X-ray CT apparatus asclaimed in claim 1, wherein said means for controlling said X-rayshielding means controls a position of said X-ray shielding means andcomprises a table that stores data on the restriction positions of saidX-ray shielding means for each predetermined rotation angle of saidX-ray irradiation means.
 3. The X-ray CT apparatus as claimed in claim1, further comprising means for controlling a position of asubject-mounting table so that a center of said concerned regioncoincides with a center of rotation of an X-ray source.
 4. The X-ray CTapparatus as claimed in claim 1, wherein said means for reconstructingan image comprises means for reconstructing said image with the use ofpreviously measured data as said outside of concerned region data. 5.The X-ray CT apparatus as claimed in claim 4, wherein said previouslymeasured data includes data obtained by a global measurement scanning.6. The X-ray CT apparatus as claimed in claim 1, wherein said means forreconstructing an image comprises means for reconstructing said imagewith the use of data obtained by calculation from data obtained by saidmeasurement in the X-ray irradiation range in addition to said dataobtained by said measurement.
 7. The X-ray CT apparatus as claimed inclaim 6, wherein said means for reconstructing an image comprises meansfor obtaining data for image reconstruction through an extrapolation ofsaid data obtained by said measurement in the X-ray irradiation range.8. The X-ray CT apparatus as claimed in claim 1, wherein said means forcontrolling a position of said X-ray shielding means comprises means forcontrolling the position of said X-ray shielding means so that, when aplurality of continuous measurements are executed, a region outside ofsaid concerned region is also irradiated with X-rays at least one time.9. An X-ray CT apparatus, comprising: a display apparatus displaying atomographic image; means for setting a concerned region on thetomographic image displayed by said display apparatus; X-ray shieldingmeans for restricting an irradiation range of an X-ray fan beam inrelation to said concerned region; means for controlling said X-rayshielding means; means for, through said X-ray shielding means,irradiating with X-rays a range including said concerned region in asubject to be inspected and executing an X-ray CT measurement; and meansfor obtaining outside of concerned region data by means of calculationand reconstructing an image with the use of said outside of concernedregion data and data of the irradiation range restricted by said X-rayshielding means, said outside of concerned region data being data of arange that is not restricted by said X-ray shielding means; wherein saidmeans for controlling a position of said X-ray shielding means comprisesmeans for controlling the position of said X-ray shielding means sothat, when a plurality of continuous measurements are executed, a regionoutside of said concerned region is also irradiated with X-rays at leastone time; and wherein said means for controlling a position of saidX-ray shielding means comprises means for setting the position of saidX-ray shielding means so that, for each scanning, the X-ray irradiationrange is gradually reduced or expanded between said concerned region anda global measurement scanning range.
 10. The X-ray CT apparatus asclaimed in claim 1, wherein said means for setting a concerned regioncomprises means for displaying a boundary of the concerned region set onsaid display apparatus.
 11. An X-ray CT apparatus, comprising: an X-raytube; a high voltage generator for supplying a high voltage to saidX-ray tube; an X-ray detector opposed to and at a predetermined distancefrom said X-ray tube; a scanner mechanism unit for rotating said X-raytube and said X-ray detector around a subject to be inspected; an imageprocessor for processing measurement data and reconstructing an image; adisplay apparatus for displaying an image reconstructed; a slicecollimator provided in an X-ray irradiation portion of said X-ray tubefor restricting an X-ray irradiation range to a slice width; X-rayshielding means provided in said X-ray irradiation portion of said X-raytube for restricting an X-ray irradiation range in an X-ray fan beamdirection; a computer for controlling operations of an entire apparatus,wherein said image processor obtains outside of concerned region data bymeans of calculation and reconstructs said image with the use of saidoutside of concerned region data and data of the irradiation rangerestricted by said X-ray shielding means, said outside of concernedregion data being data of a range that is not restricted by said X-rayshielding means; and means for controlling said X-ray shielding means sothat said irradiation range of said X-ray fan beam through said X-rayshielding means and restriction positions of said X-ray shielding meansare controlled in response to a rotation angle of an X-ray irradiationmeans.
 12. The X-ray CT apparatus as claimed in claim 11, wherein saidX-ray shielding means comprises a motor, a control unit for controllingsaid motor, and X-ray shielding boards capable of being extended orcontracted in a horizontal direction by said motor and being provided onright and left sides, respectively, with respect to an X-ray irradiationaperture.
 13. A method of reconstructing an image in an X-ray CTapparatus, comprising the steps of: obtaining projection data byexecuting X-ray irradiation with an X-ray fan beam converged onto apredetermined range by moving X-ray shielding means to predeterminedpositions in response to a rotation angle of an X-ray irradiation means;and reconstructing an image by using said projection data and correctiondata corresponding to a region outside of said predetermined range. 14.The method of reconstructing an image as claimed in claim 13, whereinsaid step of obtaining projection data comprises the steps of: obtainingfirst projection data by executing X-ray irradiation with the X-ray fanbeam converged onto a first range; and obtaining second projection databy executing X-ray irradiation with the X-ray fan beam converged onto asecond range that is narrower than said first range, and said step ofreconstructing an image comprises a step of, when said image isreconstructed on the basis of said second data, reconstructing an imageoutside of said second range by using said first data corresponding to aregion outside of said second range.
 15. The method of reconstructing animage as claimed in claim 13, wherein said step of reconstructing animage comprises a step of obtaining correction data corresponding to aregion outside of said predetermined range through an extrapolation fromsaid projection data.
 16. The method of reconstructing an image asclaimed in claim 13, wherein said step of obtaining projection datacomprises a step of obtaining projection data by converging, for eachscanning, said X-ray fan beam onto any. one of at least two ranges thatare different in size, one of the at least two ranges being narrow andthe other of the at least two ranges being wide, and said step ofreconstructing an image comprises a step of, when said image isreconstructed on the basis of projection data obtained with said X-rayfan beam converged onto the narrow range, reconstructing said image byusing, as said correction data, data of a corresponding portion ofprojection data that is the nearest in time among projection dataobtained previously with said X-ray fan beam converged onto the widerange.
 17. The method of reconstructing an image as claimed in claim 13,wherein said step of obtaining projection data comprises a step ofobtaining projection data by converging, for each scanning, said X-rayfan beam onto any one of at least two ranges that are different in size,at least said two ranges including an irradiation range in the case ofnot converging said X-ray fan beam and an irradiation range in the caseof converging said X-ray fan beam up to a concerned region, and saidstep of reconstructing an image comprises a step of, when a measurementis executed without converging said X-ray fan beam, reconstructing saidimage by using projection data obtained by said measurement, and when ameasurement is executed with said X-ray fan beam converged,reconstructing said image by using, as said correction data, data of acorresponding portion of projection data that is the nearest in timeamong projection data obtained by a previous measurement.
 18. The methodof reconstructing an image as claimed in claim 13, wherein said step ofobtaining projection data comprises the step of obtaining saidprojection data by a global measurement scanning in which said X-ray fanbeam is not restricted and a local measurement scanning in which saidX-ray fan beam is restricted onto a range of a concerned region, andsaid step of reconstructing an image comprises a step of, when saidimage is reconstructed using said projection data obtained by said localmeasurement scanning, reconstructing said image by using, as saidcorrection data, data of a corresponding portion of said projection dataobtained by said global measurement scanning.
 19. The method ofreconstructing an image as claimed in claim 13, wherein said step ofobtaining projection data comprises the steps of: setting a concernedregion on a tomographic image; and setting said concerned region as oneof said predetermined range.
 20. The method of reconstructing an imageas claimed in claim 19, further comprising, before said step ofobtaining projection data, a step of moving a position of asubject-mounting table so that a center of said concerned regioncoincides with a center of rotation of an X-ray source.
 21. An X-ray CTapparatus, comprising: a display apparatus for displaying a tomographicimage, means for setting a concerned region on the tomographic imagedisplayed by said display apparatus; X-ray shielding means forrestricting an irradiation range of an X-ray fan beam in relation tosaid concerned region; means for controlling said X-ray shielding meansso that said irradiation range of said X-ray fan beam through said X-rayshielding means is controlled in response to a rotation angle of anX-ray irradiator; means for, through said X-ray shielding means,irradiating with X-rays from said X-ray irradiator a range includingsaid concerned region in a subject to be inspected and executing anX-ray CT measurement; and means for reconstructing an image from dataobtained by said measurement, wherein said means for reconstructing animage comprises means for reconstructing said image with the use of dataobtained by calculation from data obtained by said measurement in theX-ray irradiation range in addition to said data obtained by saidmeasurement.
 22. The X-ray CT apparatus as claimed in claim 21 whereinsaid means for reconstructing an image comprises means for obtainingdata for image reconstruction through an extrapolation from said datainside the X-ray irradiation range that is obtained by said measurement.23. An X-ray CT apparatus, comprising: a display apparatus fordisplaying a tomographic image; means for setting a concerned region onthe tomographic image displayed by said display apparatus; X-rayshielding means for restricting an irradiation range of an X-ray fanbeam in relation to said concerned region; means for controlling aposition of said X-ray shielding means; means for, through said X-rayshielding means, irradiating with X-rays a range including saidconcerned region in a subject to be inspected and executing an X-ray CTmeasurement; and means for reconstructing an image from data obtained bysaid measurement, wherein said means for controlling the position ofsaid X-ray shielding means comprises means for controlling the positionof said X-ray shielding means so that, when a plurality of continuousmeasurements are executed, a region outside said concerned region isalso irradiated with X-rays at least one time, and further means forsetting the position of said X-ray shielding means so that, for eachscanning, the X-ray irradiation range is gradually reduced or expandedbetween said concerned region and a global measurement scanning range.24. An X-ray CT apparatus, comprising: a display apparatus fordisplaying a tomographic image; means for setting a concerned region onthe tomographic image displayed by said display apparatus; X-rayshielding means for restricting an irradiation range of an X-ray fanbeam in relation to said concerned region; means for controlling saidX-ray shielding means; means for, through said X-ray shielding means,irradiating with X-rays a range including said concerned region in asubject to be inspected and executing an X-ray CT measurement; and meansfor reconstructing an image from data obtained by said measurement,wherein said means for reconstructing an image comprises filtering meansfor executing a filtering processing to data in proximity to a boundaryof said concerned region among said data obtained by said measurement.25. An X-ray CT apparatus, comprising: a display apparatus fordisplaying a tomographic image; means for setting a concerned region onthe tomographic image displayed by said display apparatus; X-rayshielding means for restricting an irradiation range of an X-ray fanbeam in relation to said concerned region; means for controlling aposition of said X-ray shielding means; means for, through said X-rayshielding means, irradiating with X-rays a range including saidconcerned region in a subject to be inspected and executing an X-ray CTmeasurement; and means for reconstructing an image from data obtained bysaid measurement, wherein said means for reconstructing an imagecomprises threshold value processing means for obtaining, by means of athreshold value processing, a boundary of said concerned region of saiddata obtained by said measurement.
 26. An X-ray CT apparatus,comprising: a display apparatus displaying a tomographic image; meansfor setting a concerned region on the tomographic image displayed bysaid display apparatus; X-ray shielding means for restricting anirradiation range of an X-ray fan beam in relation to said concernedregion; means for controlling said X-ray shielding means; means for,through said X-ray shielding means, irradiating with X-rays a rangeincluding said concerned region in a subject to be inspected andexecuting an X-ray CT measurement; and means for obtaining outside ofconcerned region data by means of calculation and reconstructing animage with the use of said outside of concerned region data and data ofthe irradiation range restricted by said X-ray shielding means, saidoutside of concerned region data being data of a range that is notrestricted by said X-ray shielding means; wherein said means forreconstructing an image obtains said outside of concerned region datafrom data outside the irradiation range restricted by said X-rayshielding means and data of a region determined in view of an error ofsaid X-ray shielding means.
 27. An X-ray CT apparatus, comprising: anX-ray tube; a high voltage generator for supplying a high voltage tosaid X-ray tube; an X-ray detector opposed to and at a predetermineddistance from said X-ray tube; a scanner mechanism unit for rotatingsaid X-ray tube and said X-ray detector around a subject to beinspected; an image processor for processing measurement data andreconstructing an image; a display apparatus for displaying an imagereconstructed; a slice collimator provided in an X-ray irradiationportion of said X-ray tube for restricting an X-ray irradiation range toa slice width; X-ray shielding means provided in said X-ray irradiationportion of said X-ray tube for restricting an X-ray irradiation range inan X-ray fan beam direction; and a computer for controlling operationsof an entire apparatus, wherein said image processor obtains outside ofconcerned region data by means of calculation and reconstructs saidimage with the use of said outside of concerned region data and data ofthe irradiation range restricted by said X-ray shielding means, saidoutside of concerned region data being data of a range that is notrestricted by said X-ray shielding means; wherein said means forreconstructing an image obtains said outside of concerned region datafrom data outside the irradiation range restricted by said X-rayshielding means and data of a region determined in view of an error ofsaid X-ray shielding means.